"It was an organized massacre. The troops surrounded people, then killed them," said Rami Abdul-Rahman, head of the organization.

The killings were reported a day before Arab League observers were due to visit Syria to monitor pledges by Assad's government to withdraw troops from besieged areas.

The White House said it was "deeply disturbed" by Tuesday's attack, the State Department stepped up its travel warning, France called the deaths a "murderous spiral," and the Arab League reminded the Assad regime of its responsibilities to protect its civilians.

In a statement, the rights organization said government forces surrounded about 150 local residents and shot them with bullets and tank shells for more than five hours.

"Some women tried to break the siege but in vain," it said. "The security forces arrested a number of young people from their homes, shackled them [and] executed them," it added.

It said 111 bodies were counted in the local mosque, of which 56 had been identified by its local activists.

Dr. Mousab Azzawi, a coordinator in London for the organization, told msnbc.com that the total number of victims its local activists had verified since Monday was 228.

"The situation is absolutely getting worse by the day," he said. "This area has been crippled by protest strikes and there is no electrical power, freezing cold weather and very little communication."

The accounts could not be independently confirmed because Syria has banned entry to most foreign journalists and places heavy restrictions on the work of local reporters.

Azzawi claimed Syria's decision on Monday to allow Arab League monitors to enter the country had simply prolonged the bloodshed.

"Every time they talk it means another day without change for the people in Syria," he said.

Warnings from US
The Obama administration reacted to the latest reports by renewing its call for Assad to step down, saying he "does not deserve to rule Syria."

"The United States is deeply disturbed by credible reports that the Assad regime continues to indiscriminately kill scores of civilians and army defectors, while destroying homes and shops and arresting protesters without due process," the White House said in a statement read by spokesman Jay Carney, warning that the international community could take more steps against Syria.

State Department spokeswoman Victoria Nuland added that the stepped-up violence signaled that Syria's acceptance of the Arab League plan is merely a "stalling tactic."

"This is not the behavior of a government that is getting ready to implement the Arab League proposals," she told reporters, adding later that: "We've got lots of promises as the government continues to mow down its own people."

The department, renewing its travel warning for Syria, repeated earlier alerts that urged Americans to leave while there is still commercial air service and limit their travel inside the country due to the violence. The warning also said that already limited services at the embassy in Damascus likely would be curtailed "as staff levels ... are being further reduced."

French Foreign Ministry spokesman Bernard Valero said "everything must be done to stop this murderous spiral into which Bashar Assad is leading his people more every day." He added that the U.N. Security Council must "pass a firm resolution demanding the end to this repression."

The German government's human rights commissioner, Markus Loening, called for an immediate end to violence against deserters and demonstrators.

Turkey's Foreign Minister Ahmet Davutoglu said "it is unacceptable" that so many people were killed after Syria agreed to an Arab League plan to halt the bloodshed.

In Cairo, Arab League chief Nabil Elaraby expressed deep concern about reports of an escalation in violence and appealed to Damascus to shoulder its responsibilities to protect civilians in compliance with its pledges to abide by the league's plan.

The Arab League plan calls for Syria to halt its crackdown, open talks with the opposition, withdraw military forces from city streets and allow in human rights workers and journalists. The 22-member Arab League has also suspended Syria's membership and leveled economic and diplomatic sanctions.

Despite the new violence, the Arab League appeared to be going ahead with its plans to send in its first delegation of monitors on Thursday. An Arab League official said the second team of observers — 30 experts in military affairs and human rights — will head for Syria on Sunday, led by Lt. Gen. Mohammed Ahmed Mustafa of Sudan.

Another team of 100 observers will leave for Syria within two weeks, he said.

Syria's main opposition group, the Syrian National Council, described this weeks killings as "brutal massacres and genocide," saying it has urged the U.N. Security Council to hold an emergency meeting on Syria. The SNC also asked the international community to help protect Syrian citizens.